/Engine/Plugins/Runtime/MeshModelingToolset/Source/MeshModelingTools/Public/ToolActivities/PolyEditActivityContext.h
Gets broadcast when the CurrentMesh is modified by an undo/redo transaction emitted via EmitCurrentMeshChangeAndUpdate. Only activities that issue multiple transactions via EmitCurrentMeshChangeAndUpdate during the same invocation need to use this, since a transaction that ends immediately after the call will not have to deal with the undo.
typedef TMulticastDelegate_OneParam< void, bool bGroupTopologyChanged > FOnUndoRedo